1. Field of the Invention
This invention relates to a zoom lens suitably usable in an image pickup apparatus such as a digital still camera, video camera or TV camera, for example.
2. Description of the Related Art
Recently, the functionality of an image pickup apparatus (camera) such as a video camera or a digital still camera using a solid-state image sensing device has been raised. With the trend of such high functions of image pickup apparatuses, a small-sized zoom lens having high optical performance with a large aperture ratio covering a wide field angle (photographic field angle) has been desired.
In the cameras using a solid-state image sensing device, various optical members such as a low pass filter and a color correction filter are disposed between the lens trailing end and the image pickup device. For this reason, a zoom lens having a relatively long back focus is desired.
Furthermore, with regard to a color camera using an image pickup device for color images, the zoom lens should have good telecentric characteristic at the image side to avoid color shading.
As a zoom lens which is small in the whole system size and which has a long back focus and still which has good telecentric characteristic at the image side, a negative-lead type zoom lens having a lens group of negative refracting power disposed closest to the object side is known.
An example of such negative-lead type zoom lens is a three-group zoom lens which is comprised of, in an order from the object side to the image side, a first lens group of negative refracting power, a second lens group of positive refracting power and a third lens group of positive refracting power (Japanese Laid-Open Patent Application No. 07-261083).
A zoom lens which is one of the three-group zoom lenses and which has attempted the field-angle widening is known (U.S. Pat. No. 7,215,483 and Japanese Laid-Open Patent Application No. 2007-212636).
Furthermore, a zoom lens which is one of the three-group zoom lenses and which has attempted the zoom-ratio enhancement is known
(Japanese Laid-Open Patent Application No. 2006-208890 and U.S. Pat. No. 7,110,187).
Furthermore, a three-group zoom lens of this type in which all lens groups are moved during the zooming and chromatic aberration is well corrected using a cemented lens in the second lens group is known (Japanese Laid-Open Patent Application No. 2004-61675 and U.S. Pat. No. 7,333,275).
In order to reduce the size of the whole system and to enhance the zoom ratio in such three-group zoom lens of negative-lead type, it is effective to strengthen the refracting power of each lens group constituting the zoom lens.
However, if for example the refracting power of each lens group is just strengthened in an attempt to widen the field angle and enhance the zoom ratio, variation of aberration due to the zooming operation will increase, causing a difficulty in obtaining good optical performance in the whole zoom range.
In consideration of this, in order to accomplish reduction in size of the whole system and to achieve the field-angle widening and zoom-ratio enhancement as well in the three-group zoom lens of negative-lead type, it is very important to appropriately set the refracting power distribution of the lens groups and the movement locus of the lens groups during the zooming operation.
If, for example, the value of the refracting power of the first lens group is not set appropriately, it becomes very difficult to obtain good optical performance while assuring reduction in size of the whole system and widening of the field angle at the same time.
Furthermore, in order to obtain good optical performance in the whole zoom range while assuring the zoom-ratio enhancement, it is important that the variation of aberration should be suppressed during the zooming operation.
Particularly, in order to obtain good optical performance in the whole zoom range while assuring reduction in size of the whole lens system and zoom-ratio enhancement, it is important to set the lens structure of the second lens group which is the major power-varying lens group.
If, for example, the refracting power of the second lens group and the imaging magnification at the wide-angle end and telephoto end are not set appropriately, it becomes difficult to reduce variation of aberrations resulting from the zooming.
These are not limited to three-group zoom lenses of negative-lead type, but they are similarly applicable also to zoom lenses of negative-lead type having four or more lens groups.